The formation of scale (which can be defined as the solid precipitates that form in aqueous systems when, due to changes in the physical or chemical environment of the system, the solubility limits of certain compounds are exceeded) presents a problem in many industrial operations involving aqueous fluids, for instance oil and gas industry applications, mineral ore extraction, paper manufacture and geothermal power generation. In the oil and gas industry in particular, scale deposition on equipment surfaces may cause obstructions or blockages, leading to costly interruptions in production and safety risks from unforeseen pressure build-up. In oil and gas wells and associated equipment such as wellheads, flowlines or other processing or transportation equipment, the formation of scale is largely due to destabilisation through pressure and temperature changes in formation water and the mixing of incompatible aqueous fluids. For instance, when seawater is used as an injection fluid to drive oil through a subterranean formation towards a production well, differences in the ionic content of the injected seawater and the ionic content of the connate water of the formation can lead to the precipitation of inorganic salts. In the absence of suitable treatment, the precipitated salts form a scale which obstructs the flow of oil towards production wells and accumulates on production equipment, leading ultimately to the blockage of the production well. Similarly, precipitated salts can form scale build up on equipment associated with hydrocarbon production, processing and transportation.
Scale inhibitors are widely used in the oil and gas industry. Problems of scaling on equipment surfaces may be addressed by the continuous injection of scale inhibitors into the equipment. To prevent the formation of scale within oil/gas-bearing formations, two techniques are generally used. In one approach, a scale inhibitor may be included in a fluid (typically an aqueous fluid) to be injected into the formation via one or more injection wells, e.g. to flush oil towards a production well (water flooding treatment). In another approach, known as a “squeeze treatment”, a fluid containing a scale inhibitor (again typically an aqueous fluid) can be introduced into a production well (after production is stopped) so as to “squeeze” the scale inhibitor into the rock formation surrounding the production well. In this way, scale inhibitors are delivered to the formation rock so as to prevent the formation of scale deposits both in the formation itself (pore blockage) and subsequently in downstream production apparatus.
Scale formation can be controlled only if a scale inhibitor is provided in sufficient quantity. In the case of “squeeze treatment”, the concentration of scale inhibitor will reduce over time until a repeat treatment of the scale inhibitor is required (a “re-squeeze” treatment). It would therefore be very useful to be able to analyse the fluid produced from a production well in order to ensure that the concentration of scale inhibitor is always maintained at a level at which scale formation is sufficiently inhibited. By analysing the level of scale inhibitor in produced fluids, the depletion of scale inhibitor concentration can be monitored, and thus the need for repeat treatments of scale inhibitors can be determined. It is desirable that the level of scale inhibitors can be determined accurately, so as to avoid the need to carry out re-squeeze treatments more often than is strictly necessary, as a precautionary measure against the risk of scale deposit and consequent loss of production. Providing more scale inhibitor than is required is undesirable both due to the cost of excess scale inhibitor and due to the interruption of production that is required each time a re-squeeze treatment is carried out.
In modern oil production fields, it is increasingly common for produced fluids from a number of production wells to be combined and transported to a production facility in a single pipeline. In particular, in subsea production, it is common for the fluids from a number of production wells to be combined on the seabed, for example in a manifold, and piped to the nearest production platform, which may be many miles away. There is therefore a need for a means of analysing the level of scale inhibitor in the produced fluids from each individual well in order to ensure that individual wells do not lose production due to scale build-up. Currently, this analysis can be done in two different ways. Firstly, by turning off the flows from all but one well, the level of scale inhibitor in the one remaining well may be determined. However, this approach is not commercially viable due to the significant loss in production entailed as each individual well is tested. Furthermore, hydraulic limitations may hinder production from a single well back to a test facility. The second approach involves using different scale inhibitors in each production well, such that the level of each may be determined by analysis of the commingled flow. However, not all scale inhibitors are equally effective and, since the number of scale inhibitors required is the same as the number of wells, a situation is rapidly reached where less than optimal scale inhibitors must be used in some wells simply to ensure that each well has a different scale inhibitor. This leads to poorer scale inhibition in certain wells and therefore a requirement for more interventions in those wells than might otherwise be the case could more effective scale inhibitors be used.
It has been proposed to prepare scale inhibiting polymers which differ from one another in that they include a small number of chemical markers in the form of tagging moieties, the tagging moieties of each polymer being different from the tagging moiety of the other polymers. It is expected that, because the tagging moiety is included in the polymer in relatively small numbers, the scale inhibiting properties of the polymer will be largely unchanged from a polymer with no such tagging moieties. In this way, more than one well can be treated with an effective scale inhibitor whilst still permitting identification of the scale inhibitor in the commingled produced flow. However, the oil and gas industry has so far failed to produce a robust detection technique that accurately detects the presence and concentration of scale inhibitors in produced fluid, particularly where multiple tagged scale inhibitors are commingled and a single, optimal scale inhibitor is ideally required to treat all associated rock formations as effectively as possible.